<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <div class="change-num">
    <button @click="minus">-</button>
    <span>{{num}}</span>
    <button @click="plus">+</button>
  </div>


  <!-- 开发环境版本，包含了有帮助的命令行警告 -->
  <script src="https://cdn.jsdelivr.net/npm/vue@2/dist/vue.js"></script>
  <script>
    var app = new Vue({
      el: ".change-num",
      data: {
        num: 0
      },
      methods: {
        minus: function () {
          if (this.num <= 0) {
            alert("不能继续减了");
            return;
          }
          this.num--;
        },

        plus: function () {
          if (this.num >= 10) {
            alert("不能继续加了");
            return;
          }
          this.num++;
        }
      }
    })
  </script>
</body>

</html>